Traditionally, coil springs are designed based upon simple statics with Wahl's correction factor to compensate stress for the effect of curvature. However, this theory was derived with an assumption of zero pitch angle throughout the spring. As high strength materials were developed, coil springs were intentionally designed with fewer turns to efficiently utilize the capability of the newer high strength materials. As a result, the pitch angle of coil springs increased and the statics based simple calculations no longer provided design results with sufficient accuracy.
In 1990, Finite Element Analysis (FEA) based coil spring design, including an estimation of the spring Force Line Position (FLP—the intersection point between spring seat plane and spring reaction force vector), became popular as a way of meeting the increasing demand for higher design accuracy. Increasing computer speeds helped further the application of FEA to coil spring design methods. Since FEA is a non-reversible computation, an FEA model of a spring consisting of height and radius distribution must be created under the spring's unloaded (free) condition. The spring model is then compressed using FEA to check if the spring characteristics satisfy all design requirements such as rate, load, FLP, stress, deformed profile, etc. If the FEA results do not satisfy the design specification requirements, the free profile is modified by predicting the effect of the modification on the spring characteristics at the deformed state, as shown in FIG. 1. This modification is conducted by a designer who, through extensive experience, develops a sense of how a particular modification should effect a desired change.
The conventional FEA based coil spring design process requires extensive skill and experience for rapid and optimized design. Because of this, the resultant free profile depends on the individual designer and it is difficult to duplicate the same design every time, even by the same designer with the same requirements.
While the FEA based coil spring design method provides more ability for problem assessment at the design stage, it exacerbates design difficulties and promotes designer-dependent designs.
The present invention provides a designer-independent, rapid and robust FEA based coil spring design method as well as ideal spring shape.